The present invention relates, in general, to a method of locating the borehole of a deep well at a substantial distance below the earth's surface, and more particularly to a method for locating such a well bore by means of a magnetic field produced by current flowing in the casing of such a well bore.
It is well known in the art that in drilling deep wells for oil and gas exploration, precise control of the path followed by the well is extremely difficult, so that it is virtually impossible to know the exact location of the well at a given depth. For example, a drilling tolerance of plus or minus one quarter of a degree will allow the bottom of a 10,000 foot well to be positioned anywhere within a circle 87 feet in diameter, and numerous factors can increase this deviation. This is not of particular concern in normal drilling operations, but if an accident should occur, such as a rupture in the casing of the well which causes the well to blow out, it may become necessary to plug the well at or below the point of the rupture to bring it under control. In order to do this, a relief well must be drilled to intersect the original well at the desired level, and since such blow outs produce extremely hazardous conditions at the surface in the vicinity of the original well, the relief well must be started a considerable distance away and drilled at an incline in order to protect the personnel and the equipment used.
Because the same problems of control of the direction of drilling are encountered in the relief well as were encountered in the original well, the location of the relief well bore cannot be determined with precision; accordingly, it is extremely difficult to determine the distance and direction from the relief well to the blown out well. A further difficulty is that in some cases the force of the explosion resulting from the rupturing of a well casing can do so much damage at the surface that it may be difficult to say exactly where the well head is, much less where the well is located at a depth of 10,000 feet or more. Because of the danger at the well head, the relief well is started one-half mile or more away from the area of the original well head, angles toward the original well until it approaches relatively closely, and then follows the original well to the location where the wells are to intersect. Thus, the relief well usually is very complex, compounding the problem of knowing exactly where it is located with respect to the original well.
Numerous attempts have been made to provide accurate measurements of well location, and many surveying techniques are available which can give general information as to the location of a relief well with respect to a target well. However, such survey techniques are not capable of providing data concerning the relationship of the relief well to the original (target) well until the relief well has approached very near the original well; i.e., within a few tens of feet. At that point, equipment such as the "Magrange" (Magnetic gradient ranging) system developed by Tensor, Inc. of Austin, Texas, and Houston Oil and Minerals Research and Development, Inc., can be used with considerable accuracy; but it has been found that outside a radius of a few tens of feet, the Magrange system is usually inadequate.
In an attempt to extend the distance at which accurate information can be obtained, a variety of electrical well logging techniques have been used which treat the target well as an anomaly in the geologic structure of the earth surrounding the relief well. The art of electrical well logging is highly developed, and although systems such as the "ULSEL" (ultra-long-spaced electrode logging) system have been successfully used in locating geologic anomalies of certain types, they have been less than satisfactory in guiding relief well bores to locate and intersect existing bore holes. The ULSEL system is described extensively in the patent literature, and particularly in U.S. Pat. Nos. 3,256,480; 3,479,581; 3,697,864; and 3,778,701, all issued to Richard J. Runge. As described in these patents, this system is directed to the measurement of the apparent resistivity of the earth across a pair of electrodes, and since no directionality is given by this method, it is ineffective for directing a relief well toward an existing well.
In addition to the resistive type logging of geological strata described in the foregoing patents, there have been attempts to obtain similar data through the use of electromagnetic prospecting. Such a system is illustrated, for example, in U.S. Pat. No. 2,723,374 to Williams, which is directed to lateral exploration from a borehole to determine the magnitude and direction of resistive anomalies in the earth. Such resistive anomalies result in variations in the pattern of an electrical current injected into the earth from electrodes mounted in the same borehole as a pair of induction sensing coils mounted at right angles to each other. These sensing coils detect massive resistive anomalies such as ore bodies, salt domes, reefs, or dipping strata which have conductivities different from the conductivities of adjacent beds, and which, therefore disturb the electrical current flow. The sensing coils are used in conjunction with other conventional well logging systems to determine the probable location of mineral deposits. However, such systems do not suggest the possibility of locating relatively small targets such as well bores.
Other systems have been developed for directing a second well with respect to a first well to permit intersection of the two. One such system is described in U.S. Pat. No. 3,282,355 to Henderson, wherein a relief well is controlled to intersect a blown-out well by the use of sonic detectors responsive to the sound produced by fluids flowing out of the blown well formation. However, such a system will not operate when there is no sound emanating from the target well, and in addition, does not provide the required degree of directional and distance accuracy. Another proposal in the prior art is the use of a signal transmitter in one well and a signal receiver in the other well. For example, in patents such as U.S. Pat. Nos. 3,285,350 to Henderson, 3,722,605 to Isham, and 4,016,942 to Wallis et al., sound waves are used as the signals, while in U.S. Pat. No. 3,731,752 to Schad, the signal is a magnetic field produced by an electromagnet. In all of these systems, however, the target well must be accessible so that the signal source can be placed in one well and the receiver in the other, and they are not effective where the target well is not open, or where there is a chance of an explosion or fire that would endanger personnel.
Thus, an accurate and reliable method for determining the direction and distance of a target well from a relief well is essential, since a runaway or blown out well is not only extremely dangerous, but can cause a great deal of environmental damage. Such a method must be relatively simple, so it can be easily used under adverse field conditions, and must provide rapid and accurate results, since it is extremely important that wells that have been ruptured be quickly located and plugged, not only to protect personnel, but because the drilling of a relief well is extremely expensive and any delay in obtaining the needed information can be extremely costly.